of the partnership. If we could then take this a step further and demonstrate that those same conserved viral sequences were contributing to host survival – or, in symbiotic terms, to survival at holobiontic level – this would provide conclusive evidence for the evolutionary paradigm of virus-host symbiosis.
In fact, when we look for evidence of selection at partnership level in nature, it proves not unduly difficult to find it.
Many of my readers, whether biologists or non-biologists, will be familiar with the cruel life cycle of the parasitic wasps, where approximately 25,000 species of insects have entered into aggressive symbiotic partnerships with approximately 20,000 species of polydnaviruses. The partnership has become so intimate that many of the viruses have entered the germ line of the wasps, to emerge, as fully formed viruses, when the wasp is laying its eggs. Whether the viruses live around the wasp ovaries, or whether they emerge from the wasp genome at the time of egg-laying, they are inevitably injected into the caterpillar prey along with the wasp’s eggs. In normal circumstances the wasp’s eggs would not survive – they would be detected and destroyed by the immune system of the caterpillar. But here viral aggression comes into play, paralysing the immune system of the caterpillar, and then taking over key aspects of its internal chemistry to convert it into a brood chamber for the emerging wasp larvae. The full complexity of the symbiosis has proved to be a source of wonder to biologists, with viruses compelling the caterpillar to produce sugars to feed the larvae, and even going so far as to disrupt the caterpillar’s hormonal system, thereby preventing its natural metamorphosis into a butterfly or moth.
There can be little doubt that here we see selection operating at the level of the partnership, with viral genes and behaviour greatly enhancing the survival potential of the wasp. And when biologists, such as Provost and Whitfield, investigated the wasp-virus partnership, they found that it dated back to a single symbiotic union, approximately 74 million years ago, during the age of the dinosaurs.7
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